This invention relates generally to the application of a coating onto fibers or flat substrates. In particular, it relates to an improved method and system for measuring the interface tensile strength between a substrate and an applied coating.
Interfaces between thin films and substrates exist in many fields and applications, including composite materials, tribology, and solid state devices. In the field of composite materials, the interface between a thin coating and a fiber is considered for detecting and possibly deflecting impinging matrix cracks. In the field of tribology, interfaces between various types of functional coatings, e.g., magnetic, conducting, optical, or electrical, protective coatings, e.g., thermal barrier, corrosion, or wear resistant, or decorative coatings and their underlying substrates are of interest. In the foregoing various applications, the tensile strength of the interface is an important property that directly controls the interface decohesion process, and often controls the usefulness and reliability of the coating component. Additionally, the measurement of the interface tensile strength is of importance for reliable performance of the coating in the above applications.
Today, the laser spallation technique is used to determine the tensile strength of planar interfaces. Typically, a high energy laser pulse is made to impinge upon a planar arrangement of a confining plate, a metallic layer, a substrate plate, and a coating combination. The laser pulse impinges on a thick gold film that is sandwiched between the back surface of a substrate of interest and a fused quartz confining plate that is transparent to the wavelength of the laser. Normally, gold is used as the laser absorbing film. Absorption of the laser energy in the confined gold leads to a sudden expansion of the film which, due to the axial constraints of the assembly, leads to the generation of a compressive shock wave directed towards the test coating interface. A part of the compressive pulse is transmitted into the coating as the compression pulse strikes the interface. It is the reflection from the free surface of the coating of this compressive pulse into a tension pulse that leads to the removal of the coating, given a sufficiently high amplitude.
Prior art measurement techniques usually consist of a three-part approach. The first part was the development of a finite element computer simulation of the conversion of the laser light pulse into a pressure pulse, and of the resulting history of tensile stress which develops at the interface as the wave is reflected from the free surface of the coating. In the second part of the strategy, the pressure pulses were measured in a microelectronic, piezoelectric device in which the conditions of the computer simulation were experimentally achieved. This permitted verifying and fine-tuning the computer simulation. Finally, in the third part of the strategy, actual spallation experiments were carried out for several thin coating interfaces. The laser fluence necessary for the removal of the stressed portion of the coating at the interface was recorded, and the tensile stress across the interface that accomplished this was determined from the computer program.
However, problems exist when the prior art interface measuring technique is applied. Since the prior art method involved a finite element computer simulation of the conversion of the laser light pulse into a pressure pulse, complex nonlinear plasma equations must be solved. These equations model the multiproton ionization process within the plasma created at the film surface upon absorption of the laser energy for obtaining the optimal stress condition. These equations become more complex if a column of liquid is used as the confining medium in the experiment. The complexity of the equations that need to be solved to determine the interface tensile stress limits the types of components which can be measured. For example, samples involving substrates that can deform in a ductile fashion cannot be measured since this will require additional consideration of the modification of the stress pulse by the system components as it propagates through the substrate.
As these and other prior art techniques have proven less than optimal, an object of this invention is to measure the interface tensile stress through a direct measurement of the particle velocity at the rear surface of the coating or substrate.
Another object of the invention is to provide accurate measurements of the interface tensile stress for a variety of systems, including ductile components.
Still another object of the invention is to determine the interface tensile strength of the coating/substrate interface without necessitating the use of the complex equations.
Yet another object of the invention is to optimize the stress pulse so as to maximize the interface tensile stress to enable the system user to measure the strength of interfaces involving thin coatings.
Other general and more specific objects of the invention will in part be obvious and evidence from the drawings and description which follows.